Conductivity augmentation



31u-11 SR @ROSS Mmm Smm Room fla XR 5=139551 E June 30, 1964 E. c. LARY ETAL 3,139,551

CONDUCTIVITY AUGMENTATION Filed Feb. l, 1962 United States Patent C) 3,139,551 CONDUCTIVITY AUGMENTATION Edmund C. Lary, Vernon, Russell G. Meyerand, Jr., Glastonbury, and Robert H. Bullis, West Hartford, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Feb. 1, 1962, Ser. No. 170,486

16 Claims. (Cl. 313-63) This invention relates to a novel method for increasing the ionization of a gas. More particularly, it relates to a novel method for increasing the ionization of a gas while avoiding space charge effects.

This invention will be described as a method of increasing the conductivity of a magnetohydrodynamic generator (hereinafter referred to as an MHD generator), but it is to be expressly understood that the invention is not limited to such a use but rather than the teachings of the invention may be employed to increase the ionization of gases in general, including, but not limited to, plasma physics devices. Current state of the art practice for increasing the conductivity of an MHD generator working fluid is to seed the heated working fluid with an agent having a low ionization potential so that thermal ionization of the seeding material occurs. Alkali metals are the most favorable seeding agents known because of their low ionization potential, and cesium is the best seeding agent of the alkali metals family because it has the lowest known ionization potential.

Current reliance on thermal ionization for increasing the conductivity of MHD working fluids is severely limited by temperature considerations. At a given pressure, the conductivity of the high temperature MHD working fluid is limited by the degree of ionization of the gas or any seeding agent employed and is an extremely sensitive function of temperature. Ionization rapidly increases exponentially with increases in temperature up to a critical temperature, and then conductivity increases as a slowly varying function of temperature dominated by a 3/2 power law. Ionization is far from complete at the critical temperature, but further thermal ionization is not practical because of the diminished rate of increase of conductivity above the critical temperature. Furthermore, materials limitations make it exceedingly difficult if not impossible to pursue thermal ionization up to the critical temperature. For example, the critical temperature for cesium is about 350() K. (about 5700 F.) and about 4000 K. (about 6700 F.) for potassium, the most favorable alternative to cesium. These temperature levels exceed current materials limitations for application in MHD and plasma physics devices, and hence full advantage cannot even be taken of the exponential part of the thermal ionization process.

The present invention teaches a method whereby the degree of ionization of a conducting gas at a given temperature and pressure may be greatly increased. The method of the present invention involves exposing to the conducting gas a material, preferably in the form of fine particles suspended in the gas, part of the material being such that the surface process of thermionic emission occurs at the temperature of the gas and part of the material being such that electrons are transferred to and captured by the material from atoms or molecules in the gas by the surface process of contact ionization to produce positive ions. This is accomplished by the use of a material, part of the surface of which has a work function such that electrons are emitted at the temperature of the gas and part of the surface of which has a work function higher than the ionization potential of molecules or atoms in the gas. In this manner, the degree of ionization of the gas is greatly augmented by increasing the number of free electrons in the gas, and at the same time space charge effects are avoided through the production of neutralizing positive ions by contact ionization.

Accordingly, one feature of the present invention is a novel method for increasing the conductivity of a gas at the given temperature and pressure.

Another feature of the present invention is a novel method for increasing the conductivity of a gas through the combined use of thermionic emission and contact ionization processes.

Still another feature of the present invention is a novel method for increasing the ionization of a gas wherein particles of a material are suspended in the gas to force equilibrium of thermionic emission and contact ionization processes rather than permitting the equilibrium dictated bythe thermal ionization process.

Still another feature of the present invention is a novel method for increasing the ionization of a gas by suspending therein particles of material which either before or after suspension have a patch effect so that part of each particle emits electrons thermionically and part of each particle produces ions through contact ionization.

Still another feature of the present invention is a novel method for increasing the conductivity of a gas by suspending therein particles of material, parts of which have a work function such that thermionic emission occurs 'at the temperature of the gas and parts of which have a work function higher than the ionization potential of atoms or molecules in the gas, either naturally or by seeding, so that positive ions are produced through collisions between those atoms of the gas and the high work function part of the material.

Other features and advantages will be apparent from the following detailed description and the claims.

FIG. 1 is an illustration of an MHD generator, partly cut away.

FIG. 2 is an illustrative showing representative of a patch effect particle used in the present invention.

FIG. 3 is an illustration of a modified version of the MHD generator of FIG. 1. y

Referring to FIG. 1, the method of the present invention is employed in conjunction with the usual operation of an MHD generator 2 wherein a high temperature gas flow, such as the discharge from a combustion chamber, is subjected to the influence of a magnetic field to produce an electric potential. The gas may inherently contain atoms or molecules of low ionization potential so that a degree of thermal ionization occurs at the temperature of the gas, or the gas may be seeded with a favorable seeding agent such as an alkali metal, particularly cesium. For purposes of illustration it will be assumed in discussion that the MHD Working fluid has been seeded With cesium which is present in the gas as a vapor, some particles of which are indicated schematically at 4. Because of materials limitations it is desirable to restrict the temperature of the MHD working fluid to between 1000 K. and 2000 K. (about 1350 F. to 3100 F.), and in this range of temperature only a very small degree of thermal ionization occurs.

By the teachings of the present invention, fine particles or flakes of material 6 (see FIG. 2) are introduced into the gas stream and suspended therein by viscosity and thermal agitation. The particles are preferably metallic but may be of any material the surface of which has a work function higher than the ionization potential of parts of the gas, i.e., the cesium seeding material, and such that the work function of part of the surface of the material is lowered or can be lowered to a Value at which electron emission occurs at the temperature of the gas. With cesium being used as the seeding material, the particles can be tungsten dust particles. With present technology, metallic dust particles of 50 angstrom units thickness can be achieved. Being in suspension with the cesium vapors, these tungsten particles will become partially coated with cesium with the result that the coated areas of the tungsten dust particles have a low work function whereas the uncoated portions have a work function higher than the ionization potential of the cesium. As an alternative, fine particles of thoriated tungsten dust can be used as the particles to be suspended in the working lluid. In either event, the suspended particles have a patch effect wherein part of the surface of the material has a low work function and emits electrons thermionically at the temperature of the gas, and part of the surface of the material has a work function higher than the ionization potential of the cesium atoms. See the articles by W. B. Nottingham, Physical Review, vol. 49, page 78 (1936), and by Herring and Nichols, Reviews of Modern Physics, vol. 21, No. 2, pages 199-220 (1949), especially pages 202 et seq., for a discussion of patch effect.

Thermionic emission of electrons occurs from the low work function parts of the particles. Thus, additional electrons are made available to increase the conductivity of the working fluid. However, thermionic emission gives rise to space charge effects which ordinarliy would limit the increase in conductivity obtained thereby. It is to overcome the space charge effect that the high work function parts of the particles come into play. The agitation of the gas causes the cesium atoms to impinge on the particles of tungsten dust, and collision between the cesium atoms and those parts of the particles having a work function higher than the ionization potential of the cesium atoms results in contact ionization wherein electrons are transferred from the cesium atoms to the tungsten particles and positive ions are produced in the gas. Thus, the particles act as sites of the simultaneous production of electrons and positive ions resulting in a greatly increased ionization of the gas because the positive ions neutralize the excess electronic charge in the gas thereby eliminating space charge effects and allowing the emission of copious quantities of electrons.

Thus, a method has been disclosed for significantly increasing the conductivity of a gas by introducing and suspending in the gas particles of material having diverse work function surfaces, thereby forcing equilibrium of thermionic emission and contact ionization surface processes rather than permitting the equilibrium of the thermal ionization process. The equilibrium of the thermionic emission and contact ionization processes is assured if the characteristic recombination time between the cesium ions and the thermionically emitted electrons is long compared to the mean free time of the cesium atoms between collisions with the tungsten surfaces. It can be demonstrated that this condition can be achieved by the suspension of ultra-fine particles of the thoriated or partially cesium coated tungsten in the gas. The size and number of the particles to be used depend upon the requirement that the particles be in suspension in the gas and upon the electron density sought to be obtained in the gas. It can also be demonstrated that the use of the method of this invention with an operating temperature of the working fluid gas between 1000 K. and 2000 K. can produce an electron density 100 times greater than that which can be obtained by thermal ionization at a pressure of 1 atm. for the Working fluid and 1000 times greater than can be obtained by thermal ionization at a pressure of .0l atm. for the working fluid.

A second technique for increasing the conductivity of a gas within the teachings of this invention is the use of two types of particles in the material, one of Work function greater than the ionization potential of the seeding agent or those parts of the gas subject to thermal ionization, and the other of low work function. The latter upon thermionic emission takes on a positive potential due to a loss of electrons, thereby inhibiting further emission. In a similar way, the former assumes a negative potential resulting from the assemblage of electrons due to contact ionization. The two particle types then accomplish charge exchange through collisions so that both become electronically neutral and the surface reactions of thermionic emission and contact ionization are then repeated.

A third technique shown in FIG. 3 involves the use of a mesh of one or more pairs of electrically connected wires, one wire 8 of each pair being of a low Work function and one wire 10 of each pair being of a higher work function. The pairs of wires are inserted in the moving gas stream, and the conductivity of the gas is significantly increased through the surface processes of thermionic emission and contact ionization as described above. This third technique is limited to low pressure plasma devices because of the requirement that the mean free path for recombination be not less than the distance between the wires of a pair of wires. l

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit as defined by the followingclaims.

We claim:

l. In the method of increasing the conductivity of a gas, the step of forcing in the gas inself the simultaneous occurrence of surface process to produce ions and electrons.

2. In the method of increasing the conductivity of a gas, the step of simultaneously increasing the number of electrons in the gas by thermionic emission in parts of the gas itself and increasing the number of positive ions in the gas by contact ionization in parts of the gas itself.

3. In the method of increasing the conductivity of a gas, the steps of heating the gas, agitating the gas, and introducing into the gas particles capable of emitting electrons at the temperature of the gas and capable of simultaneously producing ions through contact with parts of the gas.

4. In the method of increasing the conductivity of a gas While avoiding space charge effects, the steps of suspending material in the gas, agitating the gas to cause collisions between the parts of the gas and the material, and heating the material, part of the material having a low work function such that electron emission occurs at the temperature to which the material is heated, and part of the material having a high work function higher than the ionization potential of at least parts of the gas so that collision between said parts of the gas and the high work function part of the material transfers electrons from said parts of the gas to the high work function part of the material and produces positive ions simultaneously with the electron emission.

5. In the method of increasing the conductivity of a gas While avoiding space charge effects, the steps of heating the gas, adding particles of material to the gas to form a mixture of particles of material suspended in the gas, and agitating the gas to cause collisions between the parts of the gas and the material, part of the suspended material having a work function such that electron emission occurs at the temperature of the gas, and part of the material having a Work function higher than the ionization potential of at least parts of the gas so that collision between said parts of the gas and the high work function part of the material transfers electrons from said parts of the gas to the high work function part of the material to produce positive ions simultaneously with the electron emission.

6. The method as in claim 5 wherein the material is of two types, one type having a low work function and the other type having a high work function.

7. The method as in claim 5 wherein the material has a patch effect.

8. The method as in claim 5 wherein the material is thoriated tungsten.

9. In the method of increasing the conductivity of a gas while avoiding space' charge effects, the steps of heating the gas, imparting motion to the gas, and introducing into the gas fine particles of material to form a mixture of the particles suspended in the gas wherein collision occurs between the parts of the gas and the particles, the surface of part of each particle having a low work function so that electron emission occurs at the temperature of the gas, and the surface of part of each particle having a work function higher than the ionization potential of at least parts of the gas so that collision between said parts of the gas and the high work function surface of the particles causes a transfer of electrons from said parts of the gas to the high work function surface to produce positive ions simultaneously with the electron emission.

10. The method as in claim 9 wherein the recombination time of ions and electrons is greater than the mean free time of said parts of the gas between collisions with the particles.

11. In the method of increasing the conductivity of a heated gas flow of low pressure, the steps of placing in the ow path of the gas at least one pair of electrically connected wires, one of the wires having a 10W work function such that electron emission occurs from said one wire at the temperature of the gas and the other wire having a high work function such that electrons are transferred to said other Wire from said gas at the temperature of the gas, and passing the gas simultaneously with the emission of electrons over the wires.

12. In the method of increasing the conductivity of a gas while avoiding space charge effects, the steps of heating the gas, imparting motion to the gas, seeding the gas with a first material of low work function to produce a vapor of said rst material suspended in the gas, the

ionization potenital of said first material being such that electron emission occurs at the temperature to which the gas is heated, introducing into said gas and forming a mixture therewith fine particles of a second material the work function of the surface of at least a part of which is higher than the ionization potential of said first material, and coating part of the surface of said particles with said first material to form areas on the surface of said particles having a low work function so that electron emission occurs from said particles at the temperature of the gas, collisions between atoms of said rst material and the high work function part of the surface of said particles causing a transfer of electrons from the atoms of said rst surface to the particles to produce positive ions simultaneously with the electron emission.

13. The method as in claim 12 wherein said first material is an alkali metal.

14. The method as in claim 12 wherein said first material is cesium.

15. The method as in claim 12 wherein said second material is tungsten.

16. The method as in claim 12 wherein said second material is thoriated tungsten.

References Cited in the file of this patent UNITED STATES PATENTS 1,648,183 Kingdon et al. Nov. 8, 1927 1,723,869 Langmuir Aug. 6, 1929 2,103,623 Kott Dec. 28, 1937 

1. IN THE METHOD OF INCREASING THE CONDUCITIVITY OF A GAS, THE STEP OF FORCING IN THE GAS INSELF THE SIMULTANEOUS OCCURRENCE OF SURFACE PROCESS TO PRODUCE IONS AND ELECTRONS. OCCURRENCE OF SURFACE PROCESS TO PRODUCE IONS AND ELE 